1. Field of the Invention
The present invention relates to an empty/load brake control device for a railway freight car and, more particularly, to a multi-stage sensing valve device to adjust the degree of brake application in accordance with load conditions of a railway freight vehicle.
2. Description of Related Art
Brake equipment for railway freight cars typically employs dual capacity empty/load equipment which adjusts the brake application force according to the empty or loaded conditions of the freight car. In such dual capacity empty/load equipment, a two-setting control is provided where normal brake pressure is realized under full load conditions and a reduced or modulated brake pressure is realized under an empty load condition. In contrast, single capacity brake equipment, which produces a brake application force independent from the load condition of the car loading, is susceptible to wheel lock and sliding wheels due to the same brake force being applied to an empty car as a loaded car. Sliding wheels undesirably cause flat spots on the wheels as well as decreased brake performance. By modulating the brake pressure under empty load conditions using dual capacity empty/load equipment, the occurrence of sliding wheels is reduced or eliminated. The changeover point between the empty and load settings is selected at a predetermined car weight, such as 20% of the full load capacity weight. When freight cars are either fully loaded or completely empty, the dual capacity empty/load equipment generally functions acceptably. However, with varying load conditions, i.e., loads that are intermediate a fully loaded and an empty condition, dual capacity empty/load type brake equipment may not be satisfactory as cars having a partial load may be under-braked or over-braked depending on the pre-selected changeover point between the empty and load settings.
U.S. Pat. No. 3,671,086 to Scott discloses a dual capacity empty/load changeover device having a load-sensing mechanism and a proportional valve. The load-sensing mechanism has a two-position spool-type valve having a first position connecting an inlet to the proportional valve and a second position connecting the inlet to atmosphere. A load-sensing arm pivots into contact with an un-sprung part of the car to position the spool valve in the first position while the car is empty and in the second position when the car is loaded. In particular, when the freight car is loaded, the load-sensing arm will contact a portion of the car to limit the travel of the load-sensing arm as well as the spool valve. When the car is empty, the spool valve will be in the first position to activate the proportional valve which modulates the brake application force applied to the wheels. The foregoing Scott patent is incorporated herein by reference.